South African patent number 2004/5318 claims a process for purifying a water-rich stream produced during a Fischer-Tropsch reaction. The process includes the use of a distillation column in which at least a fraction of non-acid chemicals (NACs) from at least a fraction of a gaseous raw product produced during a Fischer-Tropsch (FT) reaction is separated into an NAC-rich phase and an aqueous phase.
Prior to being fed to the distillation column, the gaseous raw product from the Fischer-Tropsch process is condensed and pre-treated to remove hydrocarbons in the C5 to C20 range.
This separation takes place in a three-phase separator. The three streams exiting the separator are: a tail gas, a hydrocarbon condensate including mainly hydrocarbons in the C5 to C20 range, and a so-called reaction water stream containing NACs, water, acids and suspended hydrocarbons.
The suspended hydrocarbons are subsequently separated from the reaction water stream using an oil coalescer, typically a Pall coalescer.
Should the suspended hydrocarbons contained in the reaction water stream (typically from 0.05 to 1% by mass) not be removed prior to distillation, they may cause instabilities in the distillation column or may contaminate the bottom product thereby causing said product to not meet the required specifications on hydrocarbon content.
During operation of the abovementioned process, it has been found that the mass fraction of suspended hydrocarbons in the reaction water stream is too large for the coalescer to handle, so that some hydrocarbons break through into the distillation column.
It has further unexpectedly been found that catalyst fines from the Fischer-Tropsch process remain suspended in the reaction water stream, which causes damage to process equipment and can lead to undesired side reactions in the distillation column as a portion of the catalyst fines may still be active.
The inventors therefore believe that a need exists for providing a method and an apparatus for separating suspended hydrocarbons and catalyst fines from the reaction water stream prior to feeding the stream to the distillation column.
In the specification that follows, the term “reaction water stream” is to be understood as meaning a stream derived from a Fischer-Tropsch reaction after exiting a three-phase separator as described above, including streams which have been separated therefrom e.g. in a Pall coalescer, and which reaction water stream contains NACs, water, acids, suspended solids, and suspended hydrocarbons.